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Fabrication of nanoporous silicon oxycarbide materials via a sacrificial template technique
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Fabrication of nanoporous silicon oxycarbide materials via a sacrificial template technique
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Content
FABRICATION OF NANOPOROUS SILICON OXYCARBIDE MATERIALS
VIA A SACRIFICIAL TEMPLATE TECHNIQUE
by
Xiaojie Yan
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMICAL ENGINEERING)
June 2014
Copyright 2014 Xiaojie Yan
I
Dedication
To my parents for their support, their sacrifice and the love that will never fade.
II
Acknowledgements
I would like to express my sincere regards to my advisors Professor Muhammad Sahimi
and Professor Theodore T. Tsotsis for giving me a chance to do my PhD under their valuable
supervision. I appreciate the freedom and flexibility they provided me on this research. This
dissertation is a result of their help, support, and patience. I also wish to thank Professor Thieo
E. Hogen. Esch for serving on my qualifying and dissertation committees.
I truly appreciate the help from Professor Steven R. Nutt from our department and Ms.
Regina Campbell from JEOL Ltd. for cross-section polishing of the samples. The great help
from Professor Xinhai Yu from East China University of Science and Technology on
materials characterization and the useful discussions are truly appreciated. Many thanks also
go to Mr. Mingyuan Ge for some of the TEM and SEM photos, and to Mr. Patrick Haller on
doing the DLS and for many useful discussions.
I greatly appreciate the valuable help from my good friend Dr. Wangxue Deng since the
first day I arrived at USC and all the selfless help from Dr. Yousef Motamedhashemi on
materials synthesis, system design, equipment trouble-shooting, and for teaching me how to
use the ASAP 2010, etc.
I would also like to express my thanks to Dr. Sahar Soltani, Mr. Majid Monji, Dr. Saber
Naserifar, Dr. Nitin Nair, Ms. Basabdatta Roychaudhuri, Mr. Yu Wang, Mr. Phil Kwong, Dr.
Jiang Yu, Dr. Hu Li, Dr. Qiyao Feng, Dr. Junyi Xu, Mr. Devang Dasani, Dr. Aydin Jalali, Ms.
III
Malak Khojasteh, Ms. Pavita Subanpong, Mr. Seyedhamed M Barghi, and many others in our
Group for their kind help during my research.
Special thanks also to the administrative staff members in the Department: Ms. Tina Silva,
Mr. Shokry Bastorous, Mr. Andy Chen, Ms. Karen Woo, Mr. Martin Olekszyk, Ms. Angeline
Fugelso, Ms. Laura Carlos ,Ms. Heather Alexander, Ms. Aimee Barnard, Mr. John A Curulli,
Mr. Douglas Hauser for all their help and support throughout my graduate studies
This research was made possible by financial support from the U.S. National Science
Foundation (NSF).
Finally, I would like to thank my parents, my uncles, my aunts, my cousins and all my
good friends. This thesis would never have been completed without their love and support,
especially the love and sacrifice from my beloved parents.
IV
Table of Contents
Dedication ................................................................................................................................................ I
Acknowledgements ................................................................................................................................. II
List of Tables .........................................................................................................................................VI
List of Figures ...................................................................................................................................... VII
Abstract ................................................................................................................................................... X
Chapter 1 Introduction ............................................................................................................................. 1
1.0. Background ..................................................................................................................................... 1
1.1. The Templating Method for the Fabrication of Nanoporous Inorganic Materials ..................... 2
1.2. Templated Nanoporous Inorganic Materials .............................................................................. 4
1.3. Templated Nanoporous Silicon Oxycarbide .............................................................................. 5
1.4. Nanoporous Silicon Oxycarbide from Polymer Precursor ......................................................... 7
1.5. Layered Double Hydroxides ...................................................................................................... 9
1.6. Scope of this Research ................................................................................................................. 12
References ........................................................................................................................................... 14
Chapter 2 Preparation of Silicon Oxycarbide via Chemical Vapor Deposition Using Original
Layered Double Hydroxides as Templates ........................................................................................... 24
2.0. Introduction .................................................................................................................................. 24
2.1. Experimental ................................................................................................................................ 25
2.1.1. Materials ................................................................................................................................ 25
2.1.2. Silicon oxycarbide materials prepared by chemical vapor deposition .................................. 25
2.1.3. Characterization .................................................................................................................... 26
2.2. Results and Discussion ................................................................................................................. 27
2.3. Conclusions .................................................................................................................................. 48
References ........................................................................................................................................... 50
Chapter 3 Preparation of Silicon Oxycarbide via impregnation Using Modified Layered Double
Hydroxides as Templates ...................................................................................................................... 51
3.0. Introduction .................................................................................................................................. 51
3.1. Experimental ................................................................................................................................ 53
V
3.1.1. Materials ................................................................................................................................ 53
3.1.2. Preparation of SDBS-modified MgAl-LDH ......................................................................... 54
3.1.3. Preparation of the silicon oxycarbide material ...................................................................... 54
3.1.4. Characterization .................................................................................................................... 55
3.2. Results and Discussion ................................................................................................................. 56
3.3. Conclusions .................................................................................................................................. 73
References ........................................................................................................................................... 75
Chapter 4 Future Work .......................................................................................................................... 77
References ........................................................................................................................................... 80
VI
List of Tables
Table 2.1: XPS elemental composition of the SixCyOz samples, prepared by CVD at 800
o
C by using 0.6 ml of TPS. Also shown is the excess carbon. 31
Table 2.2: The pore space properties of the samples. The triplet (x, y, z) indicates the state
of the sample before (A) or after (B) calcination, the fabrication temperature in
o
C, and the
amount of TPS used in ml. The top two samples were pure MgAl-LDH with no precursor and
no mixing (without the CVD process). In the third sample, the template, which was MgAl-
LDH, was not removed. All other samples were fabricated with the CVD process using the
TPS as the precursor, and MgAl-LDH as the template. 43
Table 3.1: XPS elemental composition of the SixOyCz samples before cross-section
polishing. Also shown is the excess carbon. 59
Table 3.2: The surface area and pore volumes of the SiO
y
C
z
samples. A indicates the
sample before calcination ,while B represents the sample after calcination. 69
VII
List of Figures
Figure 1.1: Structures of AHPCS and HPCS (Ciora et al., 2004). 8
Figure 1.2: The thermal evolution of Mg-Al-CO
3
LDH as a function of temperature. 10
Figure 2.1: The XRD spectra of the samples prepared by the CVD process at 800
o
C using
0.6 ml of TPS before calcination (A) and after calcination (B), and prepared at 700
o
C before
(C) and after calcination (D). 29
Figure 2.2: XRD spectra of MgAl-LDH template and a porous sample, prepared by CVD at
800
o
C using 0.6 ml of TPS before calcination. 30
Figure 2.3: High resolution XPS C-1s spectra of the sample prepared by CVD at 800
o
C
using 0.6 ml of TPS before calcination. 32
Figure 2.4: High resolution XPS Si-2p spectra of the sample prepared by CVD at 800
o
C
with 0.6 ml of TPS before calcination. 33
Figure 2.5: Same as in Figure 3.3, but after calcination. 34
Figure 2.6: Same as in Figure 3.4, but after calcination. 35
Figure 2.7: SEM images of the samples prepared by CVD at 800
o
C using 0.6 ml of TPS,
before calcination. (a) and (b) Show the images before polishing, while the rest represent those
after polishing. 37
Figure 2.8: Same as in Figure 3.7, but after calcination. 38
VIII
Figure 2.9: SEM-EDX results of the samples prepared by CVD at 800
o
C using 0.6 ml of
TPS, before calcination (A) and after calcination (B) after polishing. 40
Figure 2.10: Typical TEM images of the sample prepared by CVD at 800
o
C using 0.6 ml
of TPS, before calcination. (a) and (b) show two images of the same material at two different
resolutions. 42
Figure 2.11: Nitrogen sorption isotherms of the MgAl-LDH, and the sample synthesized by
CVD at 800
o
C using 0.6 ml of TPS, without removing the template. The inset shows the BJH
pore size distribution of the MgAl-LDH template, and the sample prepared by CVD at 800
o
C
using 0.6 ml of TPS, without removing the template. 46
Figure 2.12: Nitrogen sorption isotherms of the samples fabricated by CVD at 800
o
C using
0.6 ml of TPS, before (A) and after (B) calcination. 47
Figure 2.13: The HK and BJH pore size distributions of the samples prepared by CVD at
800
o
C with 0.6 ml of TPS, before (A) and after (B) calcination. 48
Figure 3.1: XRD spectra of original LDH template and LDH modified by SDBS, SDBS-
LDH. 57
Figure 3.2: XRD spectra of the samples prepared by impregnation before calcination (A)
and after calcination (B). 58
Figure 3.3: SEM-EDX elemental composition of the sample after cross-section polishing,
but before calcination. 60
Figure 3.4: SEM-EDX elemental composition of the sample after cross-section polishing
and calcination. 61
IX
Figure 3.5: High resolution XPS C 1S spectra of the sample prepared by impregnation
before calcination. 63
Figure 3.6: High resolution XPS Si 2P spectra of sample prepared by impregnation before
calcination. 64
Figure 3.7: SEM images of the sample prepared by impregnation, before calcination. (a)
and (b) show the images before polishing, while the rest represent those after polishing. 66
Figure 3.8: Same as in Fig. 3.7, but after calcination. 67
Figure 3.9: Nitrogen adsorption-desorption isotherms of the samples fabricated by
impregnation, before (A) and after (B) calcination. 70
Figure 3.10: The HK pore size distribution of the samples fabricated by impregnation,
before (A) and after (B) calcination. 71
Figure 3.11: The BJH pore size distribution of the samples fabricated by impregnation,
before (A) and after (B) calcination. 72
X
Abstract
Due to their promising mechanical properties, chemical durability, excellent oxidation
resistance, high-temperature stability (up to 1300
o
C) and other excellent properties, silicon
oxycarbide (Si
x
O
y
C
z
) materials are being used today as catalysts, catalytic supports, gas or
liquid adsorbents, lithium battery anodes, light emitters, and blood contact agents.
In this research ,layered double hydroxides (MgAl-LDHs), or modified LDHs (SDBS-
LDHs) are used as templates to fabricate nanoporous Si
x
O
y
C
z
materials using precursors ,
such as tri-isopropylsilane (TPS) and a allyl hydridopolycarbosilane (AHPCS)/
hydridopolycarbosilane (HPCS) mixture, with synthesis methods that include chemical vapor
deposition (CVD) and direct impregnation. The preparation conditions such as the pyrolysis
temperature and the CVD precursor injection flow rates are investigated. We study the
characteristics of the templated porous materials fabricated by different preparation routes
using techniques such as small-angle X-ray diffraction (XRD), wide angle XRD, X-ray
photoelectron spectroscopy (XPS), XPS elemental analysis, SEM-Energy dispersive X-ray
spectroscopy (SEM-EDX), scanning and transmission electron microscopies (SEM and TEM),
as well as nitrogen adsorption-desorption (BET) measurements.
The porous Si
x
O
y
C
z
materials fabricated from the TPS precursor and the unmodified LDH
template by CVD have a specific surface area as high as 541 m
2
/g and a total pore volume as
large as 0.91 cm
3
/g. The materials consist of fairly uniform size hollow spheres with sizes
between 10 and 30 µm and one opening, with an interior ordered layered structure and a
bimodal pore size distribution, both in the mesopore and micropore regions. The template’s
XI
ordered structure has been successfully reproduced by the CVD procedure. After calcination
of the materials in air at 450
o
C for 3 h, most of the free carbon present is eliminated and any
silicon carbide present initially has been oxidized into silicon oxycarbide.
A convenient impregnation technique has also been selected to synthesize Si
x
O
y
C
z
materials using AHPCS/HPCS polymer blends as precursors and SDBS-LDH as a sacrificial
template. The produced materials demonstrate a well-aligned, slit-like pore structure, a surface
area of 531 m
2
/g with a considerable fraction of micropores and mesopores. The key aspect of
the preparation process is the stabilization of the HPCS by the AHPCS during the thermal
treatment, which guarantees an interconnected porous space in the final material. Calcination
in air at 450
o
C for 3 h removes any free carbon from the material, while the pore structure is
preserved.
The Si
x
O
y
C
z
materials fabricated in this research, using economically-feasible templates
and precursors and convenient synthesis procedures, have desirable structures, and show
promise for applications such as catalyst supports, sorbents, membrane films for water
filtration and gas separations, and as battery anodes, etc.
1
Chapter 1
Introduction
1.0. Background
The development of efficient new techniques for the fabrication of inorganic mesoporous
(classified as having average pore sizes between 2 nm and 50 nm) and microporous (with
average pore sizes of less than 2 nm) materials has been a rapidly growing research field. This
is because such materials have been widely used for various industrial, environmental and
medical applications (Meynen et al., 2007). Microporous inorganic materials include
crystalline zeolites, as well as amorphous silicas prepared by chemical vapor deposition
(CVD), sol-gel processing techniques, etc. The current interest in microporous materials lies in
their use as sorbents, catalysts and membranes due to their ability to separate molecular
mixtures via molecular sieving, along with shape-specific molecular recognition (Brinker,
1996). Mesoporous inorganic materials have also attracted much attention over the past two
decades since the discovery of ordered mesoporous silicas (e.g., MCM-41 and similar
materials (Kresge et al., 1992)) and have been synthesized and utilized as catalysts and
catalytic supports (Viswanathan et al., 2005).
The functional properties of any porous material depend on its porosity (defined as the
volume fraction occupied by its pore space), its pore size distribution (or PSD, which is the
2
statistical or frequency distribution of the sizes of the pores), its pore connectivity (that defines
the way the pores are connected to one another), and the chemical nature of the pores’ surface
(e.g., as a result of functionalization that bestows upon the material certain properties). In
almost any application it is desired to have a porous material that has a large porosity.
However, though many porous materials do have large porosity, a large portion of it may not
be useful, because it may be isolated - not connected to the rest of the pore space - or it may be
dead-end, so that transport into and out of such pores will not be easy. Thus, it is desirable to
have porous materials that have a structure in which the pores are connected to each other, so
that isolated porosity can be minimized or completely eliminated.
In almost all applications one also requires porous materials with large pore surface area, of
the order of one to several hundred m
2
/g of the material or higher. Such a porous material must,
by necessity, have very small pores (be microporous or mesoporous), but also must have large
porosity. This necessitates the introduction of a subclass of microporous and mesoporous
materials, which we refer to as nanoporous materials. Due to their many highly important
applications, fabrication of nanoporous materials is currently pursued by several groups
around the world (Polarz et al., 2002), and is also the focus of this research project. There are
a number of techniques related to the templating method by which various types of
nanoporous materials can be prepared, which are briefly discussed below.
1.1. The Templating Method for the Fabrication of Nanoporous Inorganic
Materials
One of the relative novel and most successful ways of producing many types of nanoporous
materials is the templating method. The concept is based on using an organic or inorganic
3
compound to act as a host or structure-directing agent, in order to generate upon its removal
the void space of nanoporous materials (Polarz et al., 2002). The templating technique
typically makes use of organic and inorganic channel hosts, organic and inorganic layer hosts,
and organic and inorganic open-framework hosts (Ozin, 1992). Major synthetic templating
techniques include direct infiltration, impregnation with subsequent pyrolysis, electrochemical
deposition, electroless (i.e., chemical) deposition, in situ chemical polymerization, sol-gel
deposition, chemical vapor deposition, hydrothermal, self-assembly, etc.
The templating method can create materials that are not convenient to prepare with the
more conventional techniques. The method makes it possible to control the structure of the
final material, and it can overcome the drawbacks associated with the use of some of the liquid
precursors that are used in various fabrication methods (e.g., allyl hydridopolycarbosilane
(AHPCS) for the preparation of SiC). The templates may be classified as hard templates or as
soft templates. Normally, organic polymers or surfactants are used as the soft templates. Hard
templates are made, in turn, of inorganic materials, metals, etc. In the hard-template process,
which was developed after the soft-template method, the precursor is restricted to the surface,
cages, channels, and other pore structure of the “hard template host” (Wan et al., 2006). This
consequently will lead into materials with a 1-D, 2-D or 3-D structure that is the negative
replica or partially negative replica of the original template after subsequent thermal treatment
and template elimination. The hard-templating method can be used to prepare powders or
monolithic materials with ordered or disordered structures. Because the pore structure is
partially tunable, depending on the template characteristics (Lu et al., 2006), the hard-
templating method has attracted great interest over the past several years.
4
1.2 . Templated Nanoporous Inorganic Materials
Templated nanoporous inorganic materials include nanoporous carbon, nanoporous
silicates, nanoporous metal oxides and metals. Porous carbons prepared by the templating
method have been used in gas separations, water purification, as catalyst supports, and as
electrodes for electrochemical double-layer capacitors, and in fuel cells (Lee et al., 2006).
Generally, the production of a templated porous carbon is a five-step process: (i) synthesis of
the inorganic template; (ii) impregnation of the template with an organic precursor (such as
furfuryl alcohol, phenol-formaldehyde, or acrylonitrile); (iii) polymerization of the precursor;
(iv) carbonization of the organic material, and (v) leaching of the inorganic template
(Sakintuna et al., 2005). Various types of materials have been used as the template, such as
mesoporous silica, like SBA-15 (Jun et al., 2000), silica sol, silica gel, zeolites, clays, etc. For
example, Leroux et al. (2006) successfully synthesized porous carbons with a BET surface
area as high as 2300 m
2
/g, templated by an organo-modified layered double hydroxide (LDH);
An ordered mesoporous carbon monolith with a bi-continuous cubic structure was fabricated
by Zhao’s group using silica monoliths as hard templates (Yang et al., 2000); silica templates
prepared from sol-gel process are used to generate carbon materials of various shapes without
cracking and shrinkage (Shi et al, 2004), etc.
The most recent focus on templated silicates has been directed at mesoporous zeolitic
materials. The resulting porous materials are derived from a dissolved silica source, templated
by structure-directing surfactants. A clay with a two-dimensional layered structure can also be
used as a host to form a porous SiO
2
network (Linssen et al., 2003).
5
Another type of templated materials is porous SiC. For example, an ordered mesoporous
silicon carbide monolith has been prepared from a mesoporous silica SBA-15 template using
polycarbosilane as a precursor (Yuan et al., 2011). A variety of metal oxides, especially
ordered mesoporous oxides, have also been templated by silica or carbon, such as In
2
O
3
(Yang
et al., 2003; Tian et al., 2003), Co
3
O
4
(Tian et al., 2003; Wang et al., 2005), Mn
2
O
3
(Tian et al.,
2003), and CeO
2
(Tian et al., 2003), etc.
The templating method has been used extensively for preparing the aforementioned
nanoporous inorganic materials with high specific surface area and large porosity. However,
as it is widely known, carbon materials have low oxidation resistance; porous silicates exhibit
poor hydrothermal stability, and nanoporous metal oxide materials do not possess good
chemical inertness. Such drawbacks may all be overcome by preparing templated nanoporous
silicon oxycarbide materials, the subject matter of this research.
1.3 . Templated Nanoporous Silicon Oxycarbide
Nanoporous silicon oxycarbide is a relatively new type of porous material, with a structure
which is intermediate between silicon carbide and silica. The typical network composition for
silicon oxycarbide is [SiC
x
O
4-x
], where x may be 1, 2 or 3, in which Si, O and C are bonded
with each other in an amorphous structure (Tamayo et al., 2008). The typical black color of
the mixture is mostly due to the presence of elemental free carbon (Pantano et al., 1999).
Silicon oxycarbide possesses promising mechanical properties (Du et al., 2012), chemical
durability such as resistance to HF (Soraru et al., 2002), excellent oxidation resistance (Xu et
al., 2011; Narisawa et al., 2010), that are comparable to or even better than that of SiC
ceramics (Yuan et al., 2012), and high temperature stability up to 1300
o
C (Colombo et al.,
6
2004). Therefore, they have been used extensively in recent years as catalysts and catalyst
supports, gas adsorbents (Nghiem et al., 2006; Tamayo et al., 2011), insulation materials,
filters (Kim et al., 2004), lithium battery anodes (Ji et al., 2009; Ahn et al., 2010; Shen et al.,
2011; Liu et al., 2012; Bhandavat et al., 2013), light emitters (Karakuscu et al,. 2009; Gallis et
al., 2010), biomedical devices (Zhuo et al., 2005) such as blood contact agents (Zhuo et al.,
2005), gas sensors (Karakuscu et al., 2013), etc.
Silicon oxycarbide can be synthesized by using a sol-gel reaction process (Pantano et al.,
1999; Weinberger et al,. 2009; Ruan et al., 2010; Clark et al., 2011), laser ablation (Wang et
al., 2010), a templating method (Chan et al., 1999; Yen et al., 2005; Guan et al., 2008;
Biasetto et al., 2008; Liu et al., 2009; Xu et al., 2010), thin layer dip-coating on underlying
surfaces (Kang et al., 2011), nano-precipitation (Chen et al., 2011), emulsification
(Vakifahmetoglu et al., 2011), photo cross-linking (Martínez-Crespiera et al., 2011), etc., all
followed by direct pyrolysis.
Among the aforementioned methods, one of the most successful ones to prepare
nanoporous silicon oxycarbide materials is the templating method. For example, microcellular
Si
x
O
y
C
z
open-cell ceramic foams were fabricated (Colombo et al., 2004) from a silicone resin
using poly(methyl-methacrylate) (PMMA) microbeads as a sacrificial template; Nanotubes
made (Yen et al., 2005) of the same material have been synthesized from mesoporous
anodized aluminum oxide (AAO) with polycarbosilane as a precursor (Yen et al., 2005); while
well-aligned tubular structured Si
x
O
y
C
z
nanotubes with an ultrahigh surface area of 1387 m
2
/g
have been prepared using porous anodic alumina as templates using silicones as the precursors
(Wan et al., 2008), or by using poly(hydridomethylsiloxane) (PHMS) as the starting pre-
ceramic polymer. In addition, an ordered mesoporous Si
x
O
y
C
z
monolith with surface area as
7
high as 616 m
2
/g has been prepared with mesoporous carbon CMK-3 as a sacrificial template
(Yuan et al., 2011) using liquid PHMS as the starting pre-ceramic polymer. However, the
aforementioned methods either require expensive templates derived from time-consuming
procedures, or have not exhibited promising large surface area and desirable pore structure
properties. Moreover, none of the porous SixOyCz materials fabricated, so far, possesses an
interconnected hierarchical pore structure with both mesopores and micropores, which is the
goal of this research.
1.4 . Nanoporous Silicon Oxycarbide from Polymer Precursors
An efficient method of fabricating SiOC ceramics is based on using a pre-ceramic polymer.
Various polymers have been investigated as precursors for synthesizing silicon oxycarbide
ceramics including silicon-containing tri-block co-polymer films (Chan et al., 1999),
polycarbosilane (PCS) (Guan at al., 2008; Martínez-Crespiera et al., 2011),
polycarbomethylsilane (PCMS) (Suarez et al., 2007), silicone resin (Vakifahmetoglu et al.,
2011; Colombo et al., 1999), methyl silicone (Takahashi et al., 2003; Hassan et al., 2012),
polysiloxane (Wu et al., 2012; Duan et al., 2013; Narisawa et al., 2013),
polyhydridomethylsiloxane (Kleebe et al., 2008), 1,3,5,7-tetramethyl 1,3,5,7-tetravinyl
cyclotetrasiloxane (Bhandavat et al., 2013), among others.
The aforementioned polymers were often used in conjunction with various sacrificial
templates to fabricate silicon oxycarbide ceramics via polymer casting. Such templates
included poly(methyl methacrylate) microbeads (Kim et al., 2004; Colombo et al., 2004),
anodized aluminum oxide (Yen et al., 2005), solid-core/mesoporous-shell silica particles
(Suarez et al., 2007), SiO
2
inverse opal (Guan et al., 2008), mesoporous carbon CMK-3 (Yuan
8
et al., 2012) or such natural materials as rice bran (Hassan et al., 2012). After thermal
treatment of the resulting composite and the elimination of the sacrificial template, a pore
space with the desirable structure is generated.
In our study, three different precursors of which two are polymers, are chosen to form
nanoporous silicon oxycarbide materials. One is hydridopolycarbosilane (HCPS) and the other
one is partially-allyl-substituted hydridopolycarbosilane (AHPCS) which upon pyrolysis
yields near-stoichiometric SiC with a low oxygen content and with close to 80 % yield,
(Interrante et al., 1994), and which our group has previusly utilized to prepare SiC
microporous membranes (Ciora et al., 2004). Figure 1.1 presents the molecular structure of
AHPCS and HPCS. The other precursor is tri-isopropylsilane (TPS) which has also been used
to synthesize SiC materials by the CVD method (Sea et al., 1998), and which our group has
also previously utilized to prepare SiC microporous membranes (Ciora et al., 2004).
Figure 1.1: Structures of AHPCS and HPCS (Ciora et al., 2004).
9
1.5 . Layered Double Hydroxides
Layered double hydroxides, also known as hydrotalcites, anionic clays or pillared clays
consist of two types of metallic cations accommodated with the aid of a close-packed
configuration of OH
-
groups in a positively charged brucite-like layer. The interlayer space in
LDH is typically occupied by water and various anions for charge compensation. The general
chemical structure of LDH is
where M
II
is a divalent metal cation (such as Mg, Mn, Fe, Co, Ni, Cu, Zn, Ga) and M
III
a
trivalent metal cation (such as Al, Cr, Mn, Fe, Co, Ni, La). X
m-
represents m
-
valence
inorganic (CO
3
2-
, OH
-
, NO
3-
, SO4
2-
, ClO
4-
), heteropolyacid (such as PMo
12
O
40
3-
and
PW
12
O
40
3
), or even organic acid anions (Kim et al., 2004). Some of the functional groups that
form the LDH structure are known to be sensitive to high temperatures. The thermal evolution
of Mg-Al-CO
3
LDH as a function of temperature is shown schematically in Figure 1.2. As this
figure indicates, almost all the interlayer species leave the material by the time the temperature
reaches 405
o
C. Some studies indicate that both MgO and a stable spinel phase (MgAl
2
O
4
)
finally appear in the temperature range of 400–1000
o
C (Kanezaki et al., 1998). However,
most LDH clays can regenerate their original structure from their oxide form, when the latter
is dispersed in an aqueous solution containing the anions present in the original materials
(Costa et al., 2008). Thus, MgAl-LDH can be modified by various surfactant compounds with
the regeneration method.
LDHs have been widely used as adsorbents for liquid ions and gas molecules, as catalysts
for oxidation and reduction reactions, in novel reactive separation applications (Yang et al.,
10
2002), as well as a templates for preparing highly porous carbon, due to their well-defined
layered structure.
Figure 1.2: The thermal evolution of Mg-Al-CO
3
LDH as a function of temperature
(Yang et al., 2002).
1.5.1. Layered double hydroxides as hard templates for fabricating
nanoporous materials
Two-dimensional structured clays can be used as a host of intercalated polymer compounds
(Kyotani et al., 1988). As one of the most popular families of clay minerals, LDHs have been
used as templates for polymers/monomers penetrating into their structure. Several techniques,
including direct intercalation of the polymer itself, in-situ polymerization of various
11
monomers (Leroux et al., 2006), chemical vapor deposition (Pacula et al., 2007), and
intercalation via an anion-exchange reaction with emulsifiers such as 3-
sulfopropylmethacrylic acid (SPMA) or 2-acrylanmido-2-methyl-1-propanesulfonate acid
(Stimpfling et al., 2010) have all been used to generate the precursor/LDH composites.
The resulting materials can be used to fabricate nanoporous carbon materials. For example,
carbon replicas have been synthesized by in-situ polymerization of sodium vinylbenzene-4-
sulfonate (VBS) within the ZnAl-LDH matrix. After subsequent thermal treatment at 600
o
C
and an acid-leaching process to remove the LDH, the final porous carbon product possessed a
BET surface area as high as 2233 m
2
/g (Leroux et al., 2006). Polystyrene sulfonate
(PSS)/LDH composites have prepared by the in-situ polymerization method, and have been
used to generate porous carbons with a high BET surface area of 2300 m
2
/g, and a micropore
volume up to 1.07 cm
3
/g (Leroux et al., 2006). 4-styrenesulfonate anion (Putyera et al., 1994),
1,5-naphthalene disulfonate dianions (Putyera et al., 1996), acetonitrile (Pacula et al., 2007),
2-acrylamido-2-methyl-1-propanesulfonate (AMPS) (Stimpfling et al., 2010), 3-
sulfopropylmethacrylic acid (SPMA) (Stimpfling et al., 2010), and VBS (Prévot et al., 2011),
have also been used as precursors with LDH as the template to prepare nanoporous carbon.
The synthesis of nanoporous carbon from LDH with the CVD technique has also shown some
promise (Pacula et al., 2007). LDH is also a potentially interesting template for preparing
porous polymers. The polymerization of 12-methacryloyloxydodecanoate (MADA) ions under
UV light irradiation utilizing LDH as template results in the formation of a mesoporous ionic
polymer (Itoh et al., 2005).
12
1.6 . Scope of this Research
Nanoporous silicon oxycarbide is considered a promising material for a variety of
applications. Thus, the objective of this work is to synthesize highly porous silicon oxycarbide
materials with a large specific surface area and a bimodal, fully-connected pore structure,
using low-cost LDHs as templates and convenient synthesis routes, such as chemical vapor
deposition and impregnation, both followed by pyrolysis.
The current templating methods to synthesize nanoporous silicon oxycarbide are not able to
synthesize materials with hierarchical, fully-connected pore structures. Either the materials do
not possess high surface area or they need expensive templates with tedious synthesis
procedures. The choice of the templating method is because it is one of the most successful
methods for preparing nanoporous materials, and LDH as a two- or quasi-two-dimensional
material is potentially a useful template for preparing nanoporous silicon oxycarbide ceramics.
However, to the best of our knowledge, no other study on synthesizing nanoporous silicon
oxycarbide materials using LDH as a template has ever been carried out before.
In this study, therefore, we aim to study the ability of the templating technique to prepare
nanoporous silicon oxycarbide materials and to carefully investigate the impact of the
synthesis conditions, like precursor amounts, heating temperature, etc., with the various
fabrication methods on the properties of the resulting materials.
13
The thesis is organized as follows:
Chapter 2 investigates the possibility to synthesize highly porous Si
x
O
y
C
z
materials by
chemical vapor deposition using LDH with particle size less than 25 µm. XRD and XPS are
conducted to confirm that the materials are Si
x
O
y
C
z
. The morphology and the ordered interior
structure are studied by SEM and TEM. XPS elemental analysis and SEM-EDX are applied to
understand the composition changes of the materials before and after calcination at 450
o
C. N
2
sorption measurements were also used to compare porous materials prepared under different
pyrolysis temperature of 700
o
C and 800
o
C (this research is already published in Yan et al.,
2014).
In Chapter 3, another technique, namely direct impregnation was selected combined with
using SDBS-modified LDH as a template to tailor the final Si
x
O
y
C
z
materials’ pore structure.
The fact that both the LDH lamellar distance has been enlarged by SDBS, and the radius of
gyration of the polymer precursor is reduced by the AHPCS/HPCS mixing, makes the
intercalation of the precursor into the LDH structure possible. The nature of the material has
been studied by XRD and XPS. SEM-EDX confirmed that calcination in air removed much of
the free carbon from the original silicon oxycarbide materials produced, with a small amount
of oxygen introduced. SEM images have been generated to illuminate the materials’
morphology. The materials have high surface area with a large fraction of micropores whose
existence is confirmed by N
2
adsorption measurements (this research is presented in Yan et al.,
to be submitted for publication 2014).
In Chapter 4 some suggestions on future work are discussed.
14
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24
Chapter 2
Preparation of Silicon Oxycarbide via Chemical Vapor Deposition
Using Original Layered Double Hydroxides as Templates
2.0 . Introduction
In this Chapter we present our results on the successful fabrication of highly porous silicon
oxycarbide materials with high surface area, a hierarchical and ordered layer pore structure by
using chemical vapor deposition with tri-isopropylsilane (TPS) as the precursor and low cost
MgAl-layered double hydroxide (MgAl-LDH) as the sacrificial template, via the convenient
route of chemical vapor deposition (CVD) at relatively low temperatures of 700
o
C to 800
o
C.
Due to the probability of free carbon existing in the Si
x
C
y
O
z
material produced by the
technique, part of the powder was calcined in a muffle oven in an air atmosphere at 450
o
C for
3 h to eliminate the elemental carbon. Different preparation conditions such as the TPS
amounts injected and the final pyrolysis temperature were investigated, and the pore structure
characteristics of the materials were compared.
The final materials produced by using the TPS and LDH as template by CVD, were
characterized by wide angle XRD, and X-ray photoelectron spectroscopy (XPS). XPS
elemental analysis and SEM-EDX were also conducted on the samples before and after
calcination. Characterization of the structure and the morphology of the porous materials both
before and after calcination were carried out using scanning electron microscopy (SEM). The
ordered structure of the material was studied by both small-angle XRD and TEM. Nitrogen
25
adsorption-desorption isotherms were also generated and used to identify the specific surface
area, the pore size distribution, the pore volume and other characteristics of the resulting
porous material produced under various preparation conditions.
2.1. Experimental
In what follows, we first describe the experiments and then present and discuss the results.
2.1.1. Materials
The TPS (from Sigma-Aldrich Co.) and the hydrochloric acid (37 wt. %, from VWR
International) were used without further purification. The MgAl-LDH powder was provided
by the Sasol Corporation (Sasol Mg 70 with a Mg/Al ratio of 3.0).
2.1.2. Silicon oxycarbide materials prepared by chemical vapor deposition
First, the portion of the MgAl-LDH powder with particle sizes less than 25 µm was
separated using a 500-mesh sieve. Next, the LDH powder was placed in a tube furnace heated
at a rate 1
o
C /min. When the temperature of the furnace reached 120
o
C, a mixture of argon
containg TPS at a certain fixed concentration was flown through the tube furnace. The LDH
sample temperature was then increased (1
o
C /min) in the flowing TPS/Ar mixture until it
reached either 700
o
C or 800
o
C, and was then held at the target temperature for 3 h. The
heater was then turned-off in order for the system to cool down in an argon gas atmosphere to
ambient temperature. The resulting dark brown powder was then dipped into an 18 wt.% HCl
solution to remove the template. The mixture was then vigorously stirred for 48 h before the
26
powder was filtered and washed with de-ionized water (DIW) to remove any potential soluble
impurities. After drying overnight at 90
o
C, a fine black powder was obtained. Part of this
powder was calcined in a muffle oven in an air atmosphere at 450
o
C for 3 h in order to
eliminate any elemental carbon that may be present. Further characterizations were then
conducted on both the uncalcined and calcined samples, the results of which are described
shortly.
2.1.3. Characterization
The characterization began with X-ray diffraction analysis, which was carried out using a
Rigaku X-ray diffractometer, with the CuK
α
line used as the X-ray source with a
monochromator positioned in front of the detector. Scanning was performed over angles, 2θ,
ranging from 10° to 90°, or from 0.2° to 50° with a scanning rate of 4° /min and stepwise
increase of 0.05. X-ray photoelectron spectroscopy measurements were performed using a
Thermo ESCALAB 250 spectrometer. A monochromatic AlKa radiation source (µν ~ 1486.6
eV) was used at a spot size of 400 µm. A pass energy of 100 eV for the survey scan and 20 eV
for the high resolution scan was used with the analyzer. Low-resolution survey scans were
acquired at binding energies between 1 eV and 1400 eV with a resolution of 1 eV. High-
resolution scans of the C-1s and Si-2p regions were acquired with a resolution of 0.05 eV.
Data analysis was performed using the XPSPEAK, Version 4.1, software.
The samples, before and after sectioning, were examined by scanning electron microscopy
with a 6010LA variable pressure W-SEM, using secondary electron imaging (SEI). Before
imaging, the powder was sprinkled on a copper tape. The sample was sectioned by a JEOL
cross-section polisher, utilizing an argon-ion beam at 6 kV, at a milling rate of 100 µm/h for
27
25 min. The samples before and after sectioning were analyzed using a JEOL JSM 6360M
scanning electron microscope equipped with an EDAX FALCON energy dispersive X-ray
spectrometry (EDX) unit. Analysis of the morphology was also carried out utilizing
transmission electron microscopy, using JEOL TEM 2100 LaB6 under room temperature, 220
kV acceleration voltage and a bright field illumination.
Nitrogen adsorption - desorption isotherms in the porous samples were measured with a
Micromeritics ASAP 2010 adsorption analyzer at -196
o
C (77 K). Prior to the measurements,
the porous silicon oxycarbide materials were degassed overnight at 110
o
C. The Brunauer -
Emmett - Teller (BET) method was utilized to calculate the surface areas. The pore volume
and pore size distributions were derived from the isotherms using the Barrett
- Joyner -
Halenda (BJH), as well as the Horvath - Kawazoe (HK) methods.
2.2. Results and Discussion
Four samples were prepared at 800
o
C, and another four at 700
o
C. In each case, two
samples were fabricated using 0.6 ml of TPS, and two with 1.0 ml of TPS as the precursor. In
addition and for the sake of comparison, we also measured the properties of the pure MgAl-
LDH template, both at room temperature and at 800
o
C. As expected, since the LDH materials
collapse at temperatures much lower than 800
o
C (Yang at al., 2002), the template had no
useful properties after being treated at such a high temperature. If the template is not removed
at the intended temperature of 700
o
C or 800
o
C, the porous material will still not possess
useful properties. But, the important point to emphasize is that although the LDH materials are
not useful at high temperatures, they can still be used as a template for fabrication of the type
of porous materials that we describe in this Chapter and the following Chapter, because they
28
are inexpensive, they represent an attractive alternative to other types of materials used in the
past for fabricating porous materials.
Figure 2.1: The XRD spectra of the samples prepared by the CVD process at 800
o
C using
0.6 ml of TPS before calcination (A) and after calcination (B), and prepared at 700
o
C before
(C) and after calcination (D).
Figure 2.1 presents the XRD data for four samples prepared by CVD, two each at the two
high temperatures. All the samples display the typical noisy spectra for amorphous materials.
No typical template peaks corresponding to MgO, Al
2
O
3
, or MgAl
2
O
4
can be identified, hence
indicating that the template has been removed completely. The XPS elemental analysis, to be
described below, verified this conclusion. The broad diffraction peak at about 2θ = 21° - 24° is
29
attributed to silica oxycarbide glass. For one sample, referred to as sample A in Figure 2.1 that
was fabricated with 0.6 ml of TPS at 800
o
C without calcination, the (200) peak, which is
attributed to β-SiC, is in the spectrum, whereas for sample B that was prepared under similar
conditions but was also calcined, the corresponding peak is much weaker. This is presumably
because β-SiC was formed at 800
o
C, but was transformed to Si
x
C
y
O
z
during the calcination
process in air. The XRD spectra of the materials produced at 700
o
C and shown in Figure 2.1
do not also indicate a strong peak corresponding to β-SiC.
Figure 2.2: XRD spectra of MgAl-LDH template and a porous sample, prepared by CVD
at 800
o
C using 0.6 ml of TPS before calcination.
30
In Figure 2.2 we compare the spectra of sample A of Figure 2.1 and the MgAl-LDH
template at room temperature. The comparison indicates a sharp peak at 2θ = 1.28°,
corresponding to Si
x
C
y
O
z
after removing the MgAl-LDH template. Such a low angle peak is
indicative of the existence of a layered structure, corresponding to the basal reflection of (003).
The calculated basal spacing d
003
of sample 1 is 6.9 nm, which is further confirmed by the
TEM data, to be described below. This should be compared with the basal spacing of 0.76 nm
for the original MgAl-LDH template. Thus, not only the layered structure that should have
been destroyed at 800
o
C has been stabilized by the deposition of TPS, but the corresponding
vacancy has also been maintained even after the template’s removal.
Table 2.1 XPS elemental composition of the Si
x
C
y
O
z
samples, prepared by CVD at 800
o
C
by using 0.6 ml of TPS. Also shown is the excess carbon.
Samples
Atomic ratio
Composition
Si C O
Before calcination 1 3.7 1.7 SiO
1.7
C
2.3
+C
1.4
After calcination 1 2.4 1.9 SiO
1.9
C
2.1
+C
0.3
The XPS elemental compositions of the Si
x
C
y
O
z
samples, prepared by CVD at 800
o
C with
0.6 ml of TPS are shown in Table 2.1, along with the composition of Si
x
C
y
O
z
for which x = 1.
Before calcination in air, a certain amount of excess (also known in the literature as “free” or
“elemental”) carbon exists, since the ideal atomic ratio (C + O)/Si should be around 4. (Note
though, that the actual value of y could be less than 1.7, as shown in Table 2.1, for a trace
amount of oxygen may chemisorb on the material during the tests). After calcination in air,
31
part of the free carbon is removed, as demonstrated in Table 2.1, and thus the carbon content
of the material has decreased. On the other hand, after calcination the value of y corresponding
to oxygen has shifted to a higher value. This illustrates the possibility that, due to calcination
in air, a certain fraction of the SiC and Si
x
C
y
O
z
is oxidized. The atomic ratio (C + O)/Si
decreases to 4.3, which is a typical value for silicon oxycarbide materials with elemental
carbon. These results verify that all, or at least most of the free carbon is oxidized during
calcination. After calcination the excess amount of carbon, as shown in Table 2.1, may be due
to chemisorbed CO
2
, and/or by bonded C - C as it is also shown in the high resolution C-1s
XPS results, in Figure 2.3 below (Pantano at al., 1999). In summary, the elemental analysis
indicates that the Si, O and C stoichiometry is within a reasonable range for Si
x
C
y
O
z
materials.
As the XRD data indicated, SiC is also formed during the CVD process.
288 286 284 282 280
0
1000
2000
3000
4000
5000
6000
Intensity (a.u.)
Binding energy (ev)
C-Si
C-C
C-O
Figure 2.3: High resolution XPS C-1s spectra of the sample prepared by CVD at 800
o
C
using 0.6 ml of TPS before calcination.
32
106 104 102 100 98
0
200
400
600
800
1000
SiC4
Intensity (a.u.)
Binding energy (ev)
OxSiCy
SiO4
Figure 2.4: High resolution XPS Si-2p spectra of the sample prepared by CVD at 800
o
C
with 0.6 ml of TPS before calcination.
A considerable amount of valuable information about the bonding structure and the
composition is obtained from the XPS measurements. Thus, in order to better understand the
composition of the materials, a high resolution XPS scan of C-1s and Si-2p on the
aforementioned samples A and B of Figure 2.1, prepared at 800
o
C with 0.6 ml of TPS, before
and after calcination, was conducted. Curve fitting of the XPS spectrum for C-1s of the SiOC
in sample 1 is shown in Figure 2.3. The C-1s peak at 284.6 eV is due to the C-C bonds of the
free carbon. The main peak at 283.5 eV corresponds to the C-Si bonds (Onneby et al., 1996),
and a shoulder centered at 286.5 eV is potentially due to the C-O bonds in the adsorbed CO
2
33
(Yuan et al., 2012). It is clear that the C-Si bonds are dominant. Thus, most of the C atoms in
the materials are bonded to Si, forming silicon oxycarbide and SiC. However, even the C-C
bonds may be connected with the silicon oxycarbide structure stably (Pantano et al., 1999),
which is also verified below (see Figure 2.5).
290 288 286 284 282 280
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Intensity (a.u.)
Binding energy (ev)
C-Si
C-C
C-O
Figure 2.5: Same as in Figure 2.3, but after calcination.
34
106 104 102 100 98
0
200
400
600
800
1000
1200
1400
Intensity (a.u.)
Binding energy (ev)
SiO4
OxSiCy
Figure 2.6: Same as in Figure 2.4, but after calcination.
The fitted curves of the Si-2p spectra shown in Figure 2.4, which are for the sample before
calcination, indicate that the Si-2p peak at 100.0 eV is the typical peak for Si-C bonds (Yuan
et al., 2012). The peak at 102.5 eV corresponds to the Si-C bonds in all the three silicon
oxycarbide network species, namely, [SiOC
3
], [SiO
2
C
2
], and [SiO
3
C] (Karakuscu et al., 2009),
whereas the 103.2 eV peak is typical for Si–O bonds of [SiO
4
] (Karakuscu et al., 2009).
Although SiO
2
does exist, Si
x
C
y
O
z
and SiC dominate, as can be estimated from corresponding
peak area.
Shown in Figure 2.5 are the high-resolution C-1s spectra for the sample B of Figure 2.1,
calcined in air. One may discern that the peaks for the three types of carbon bonds, namely, C-
35
Si, C-C and C-O, still exist. Since the elemental analysis indicates that most of the free carbon
has been removed, the explanation is that, due to the oxidation during calcination, the amount
of C-Si bonds also decreases, as SiC is oxidized mostly into Si
x
C
y
O
z
and, in turn, a small
amount of the latter material is oxidized and transformed into SiO
2
.
The corresponding Si-2p spectra for the same sample are shown in Figure 2.6. They
indicate that the peak corresponding to the Si-C bond disappears, implying that Si-C has been
oxidized into silicon oxycarbide. This is consistent with the same samples’ XRD data shown
in Figure 2.1 that, as discussed, indicate that after calcination the peaks that correspond to β-
SiC become indistinguishable. The three main peaks at 101.5 eV, 102.1 eV and 102.6 eV all
belong to the Si-C bonds in the aforementioned molecular species, namely, [SiOC
3
], [SiO
2
C
2
],
and [SiO
3
C].
36
Figure 2.7: SEM images of the samples prepared by CVD at 800
o
C using 0.6 ml of TPS,
before calcination. (a) and (b) show the images before polishing, while the rest represent those
after polishing.
37
Figure 2.8: Same as in Figure 2.7, but after calcination.
38
Figure 2.7 presents the SEM images of the porous materials. Images (a) and (b) of sample
A of Figure 2.1 indicate that its particles before calcination were homogeneous hollow spheres
with one opening on each sphere. The sphere sizes are in the range 10 - 30 µm. The polished
images of the cross sections of the same sample, shown in Figure 2.7(c) - (f), demonstrate that
the hollow spheres have a layered internal structure. Since the XRD results, combined with the
elemental analysis, indicate the complete removal of the LDH template, the images seem to
confirm that the interior structure of the particles are negative replicas of the LDH layered
structure, which also verifies that the TPS molecules have penetrated the interlayer structure of
the LDH template. This result is completely consistent with the experimental expectation that
the TPS both wraps and penetrates into the template’s layers during the CVD process, since
the entire process begins at 120
o
C at which the LDH structure also begins its transformation.
Figure 2.8 shows SEM images of the porous material after air calcination, denoted by B in
Figure 2.1. After the free carbon has been eliminated, the structure partially collapses and the
hollow structure of the spheres is destroyed to some extent, although the main frame still
exists. Figure 2.8(c) - (f) also show a clearer view of the layered structure after the calcination.
39
Figure 2.9: SEM-EDX results of the samples prepared by CVD at 800
o
C using 0.6 ml of
TPS, before calcination (A) and after calcination (B) after polishing.
The SEM-EDX results as shown in Figure 2.9 provide further comparison of the porous
materials’ interior composition before calcination (A from Figure 2.1) and after calcination (B
from Figure 2.1) after cross-section sectioning. After calcination in air, less carbon has been
detected from the materials’ interior surface from various selected areas, as shown in the
embedded tables from Figure 2.9, which verifies the fact that the free carbon has been partially
eliminated.
The SEM images, combined with the XRD and XPS results, demonstrate that the
calcination does eliminate most of the free carbon but, at the same time, introduces a little bit
40
more oxygen into the material. Moreover, during the oxidation process, the materials’ original
structure is partially destroyed, although their specific surface area does not change, an
assertion that is confirmed based on nitrogen adsorption - desorption data that we describe
shortly.
Figure 2.10 presents the TEM images of the porous material fabricated at 800
o
C, before it
was calcined. The images clearly show the ordered layered structure of the sample, which can
be discerned only from certain directions due to the small domain of the layered structure of
the template and the overlay of the template during the preparation of the porous material. The
distance between two neighboring layers or the height of the interlayer space, as estimated
from Figure 2.10 (b), is around 6 - 7 nm, which agrees well with the XRD results. Thus, the
CVD process expands the interlayer distance of the LDH template from its original value of
0.76 nm to around 7 nm. We may explain this by observing that during the CVD process TPS
is injected, beginning at 120
o
C and, at the same time, the LDH template begins to lose the
interlayer species and deforms. After the interlayer bonds between various species are broken,
the TPS molecules penetrate the interlayer space continuously and swell the template. Thus,
one may be able to control the interlayer region of Si
x
C
y
O
z
that may acts as a mesopore. This
is currently under further study.
41
Figure 2.10: Typical TEM images of the sample prepared by CVD at 800
o
C using 0.6 ml
of TPS, before calcination. (a) and (b) show two images of the same material at two different
resolutions.
42
Table 2.2: The pore space properties of the samples. The triplet (x, y, z) indicates the state
of the sample before (A) or after (B) calcination, the fabrication temperature in
o
C, and the
amount of TPS used in ml. The top two samples were pure MgAl-LDH with no precursor and
no mixing (without the CVD process). In the third sample, the template, which was MgAl-
LDH, was not removed. All other samples were fabricated with the CVD process using the
TPS as the precursor, and MgAl-LDH as the template.
Sample Appearance
S
BET
(m
2
/g)
V
total
(cm
3
/g)
V
meso
(cm
3
/g)
MgAl-LDH White powder 16.2 0.09 0.08
MgAl-LDH at
800
o
C
White powder 0.65 0.01 0.00
(N/A, 800,0.6) Fine black powder 13.0 0.06 0.05
(A, 800, 1.0) Fine black powder 363 0.70 0.59
(B, 800, 1.0) Fine black powder 330 0.50 0.39
(A, 800, 0.6) Fine black powder 402 0.69 0.57
(B, 800, 0.6) Fine black powder 541 0.91 0.73
(A, 700, 1.0)
Fine black /brown
powder
366 0.73 0.61
(B, 700, 1.0)
Fine black /brown
powder
340 0.74 0.50
(A, 700, 0.6)
Fine black /brown
powder
418 0.91 0.77
(B, 700, 0.6)
Fine black /brown
powder
400 0.79 0.65
43
The surface area and porosity of the samples were studied by nitrogen adsorption -
desorption measurement. Table 2.2 summarizes the results, including those of the LDH
template, a porous sample without removing the template, and the Si
x
C
y
O
z
materials prepared
by CVD at 700
o
C and 800
o
C, using 0.6 or 1 ml of the TPS. The MgAl-LDH template has a
relatively low BET specific surface area of 16.2 m
2
/g and low total pore volume of 0.09 cm
3
/g.
After the TPS was deposited on the surface and penetrated inside the LDH layered structure,
the surface area of LDH-Si
x
C
y
O
z
composite did not change significantly when compared with
that of the pure template. Moreover, the template that is treated at 800
o
C without TPS
injection loses the surface area almost completely. These results illustrate that MgAl-LDH
itself is not a promising porous material with or without the CVD process. In addition to the
TEM and SEM results, the nitrogen adsorption - desorption isotherms also verify that the
pores have a slit-like structure.
Figure 2.11 presents the nitrogen adsorption - desorption isotherms, along with the BJH
pore (volume) distribution shown in the inset. They reveal that, although both the MgAl-LDH
template and the LDH- Si
x
C
y
O
z
composites, prepared at 800
o
C, exhibit type-IV hysteresis
loops, a H3 hysteresis loop is obtained for MgAl-LDH, whereas for LDH- Si
x
C
y
O
z
it is more
closely associated with the H4 hysteresis loop (Sing et al., 1985). This is attributed to the TPS
deposition, because the fabricated Si
x
C
y
O
z
contains narrow and uniform pores due to the
aggregation of plate-like particles. It is also in reasonable agreement with the pore size
distribution shown in Figure 2.11, which illustrates a narrow distribution for the LDH-
Si
x
C
y
O
z
material, prepared by CVD at 800
o
C using 0.6 ml of TPS, and should be compared
with the wide pore size distribution of the pure MgAl-LDH.
44
After the removal of the template, the synthesized Si
x
C
y
O
z
consists of mesopores with a
type IV-H4 hysteresis loop, and a bimodal pore size distribution. Figure 2.12 compares the
sorption isotherms for the same porous material as in Figure 2.11, before and after calcination,
while Figure 2.13 presents their corresponding pore volume distributions, obtained by the HK
and BJH methods. The HK results in Figure 2.13 should be compared with the BJH pore
volume distributions, which are also shown in Figure 2.13. The isotherm characteristics of the
two samples before and after calcination are consistent with those of LDH-Si
x
C
y
O
z
, prepared
by CVD at 800
o
C using 0.6 ml of TPS and calcined, hence suggesting that part of the
structure had formed before the template was removed. Once again, this indicates that the
intercalation of the precursor stabilizes the LDH structure, since at 800
o
C the slit pores still
exist, which is in contradiction with what is expected of the thermal evolution of LDH
materials (Yang et al., 2002) that indicates that their structure should collapse at around 450
o
C.
The data listed in Table 2.2 indicate the Si
x
C
y
O
z
samples have a specific BET surface area
that is anywhere between 330 m
2
/g and 541 m
2
/g. Comparison with the relatively low surface
area of the same sample before calcination indicates that the CVD process and template
removal both contribute to the large surface area of the final porous material. Since the
template itself does not have a promising specific surface area, this must be attributed to the
complex template evolution process, combined with the TPS deposition and the intercalation
effect during the CVD process.
Table 2.2 also indicates that, when the TPS amount is lowered, the specific BET surface
area and total pore volume of the Si
x
C
y
O
z
samples both increase, hence indicating that higher
TPS amounts cause the densification of the porous materials. However, the effect of
temperature on the structure of the samples is not quite clear. For example, as the deposition
45
temperature decreases from 800
o
C to 700
o
C, no difference is detected between the specific
surface areas of the two samples. Thus, one might conclude that the amount of the precursor is
more influential in the formation and properties of the final porous material after calcination.
Indeed, after calcination in air, the pore surface area of the sample, prepared with 0.6 ml of
TPS, increases from 380 m
2
/g to 541 m
2
/g, while the total pore volume increases from 0.69
cm
3
/g to 0.91 cm
3
/g, which presumably is due to the formation of a large number of small
pores.
Figure 2.11: Nitrogen sorption isotherms of the MgAl-LDH, and the sample synthesized
by CVD at 800
o
C using 0.6 ml of TPS, without removing the template. The inset shows the
BJH pore size distribution of the MgAl-LDH template, and the sample prepared by CVD at
800
o
C using 0.6 ml of TPS, without removing the template.
46
Figure 2.12: Nitrogen sorption isotherms of the samples fabricated by CVD at 800
o
C
using 0.6 ml of TPS, before (A) and after (B) calcination.
47
Figure 2.13: The HK and BJH pore size distributions of the samples prepared by CVD at
800
o
C with 0.6 ml of TPS, before (A) and after (B) calcination.
All the results, when combined with XRD, indicate tentatively that a deposition
temperature of 800
o
C, together with an injection amount of 0.6 ml of TPS, represent optimal
conditions for the fabrication of our porous materials. For all the Si
x
C
y
O
z
samples, the specific
BET surface area is larger than that of the MgAl-LDH template by a factor of at least 20, as
well as that of LDH-Si
x
C
y
O
z
composites before calcination.
48
2.3. Conclusions
We have reported here on the preparation of nanoporous materials with a specific surface
area as high as 541 m
2
/g and total pore volume as large as 0.91 cm
3
/g. The materials have a
hierarchical pore structure with a narrow pore size distribution, both in the meso- and
micropore regions. They were prepared by chemical vapor deposition, using layered double
hydroxides as a sacrificial template. Compared to the original template, both the specific
surface area and the total pore volume of the materials increase tremendously. The materials,
representing silicon oxycarbide with silicon carbide and elemental carbon also being present,
contain homogeneous hollow spheres with sizes between 10 and 30 µm and one opening, with
an interior ordered layered structure. The proposed structure was verified by SEM, TEM and
nitrogen sorption measurements. After calcination in air at 450
o
C for 3 h, most of the free
carbon and all of the silicon carbide disappear, as indicated by high resolution Si-2p and C-1s
XPS spectra. The highly porous materials with a hierarchical narrow pore size distribution,
ordered slit-like pores and unique properties, such as high temperature stability and chemical
durability, were fabricated using economically-feasible templates and precursors, a simple
CVD system and convenient synthesis procedures.
As such, the new materials have several potential applications. For example, as they
contain both meso- and micropores, they provide sufficient surface for active catalyst sites to
be deposited on, as well as a range of pore sizes that is suitable for gas-phase compounds to
pass through. Thus, the materials have high potential as candidate for catalysts’ support. Due
to their high porosity/high surface area, the materials are suitable for H
2
storage, given the fact
that the excess carbon can play a positive role during sorption of the gases. The layered
structure of the materials is of interest to the researchers that are looking for an alternative for
49
graphite in lithium batteries, since our material does not degrade the way graphite does.
Finally, the precursor is cheap and highly economical.
50
References
Karakuscu, A., Guider, R., Pavesi, L. and Soraru, GD. White luminescence from sol-gel-
derived SiOC thin films. Journal of the American Ceramic Society, 2009, 92 (14): 2969-2974.
Onneby, C. and Pantano, CG. Silicon oxycarbide formation on SiC surfaces and at the
SiC/SiO
2
interface. Journal of Vacuum Science & Technology A-Vacuum Surfaces and Films,
1996, 15 (3): 1597-1602.
Pantano, CG., Singh, AK. and Zhang, HX. Silicon oxycarbide glasses. Journal of Sol-Gel
Science and Technology, 1999, 14 (1): 7-25.
Sing, KSW., Everett, DH., Haul, RAW., Moscou, L., Pierotti, RA., Rouquerol, J. and
Siemieniewska, T. Reporting physisorption data for gas/solid systems with special reference to
the determination of surface area and porosity. Pure and Applied Chemistry, 1985, 57 (4):
603-619.
Yang, WS., Kim, Y., Liu, PKT., Sahimi, M. and Tsotsis, TT. A study by in situ techniques
of the thermal evolution of the structure of a Mg-Al-CO
3
layered double hydroxide. Chemical
Engineering Science, 2002 (57): 2945-2953.
Yuan, XY., Jin, HL., Yan, XB., Cheng, LF., Hu, LT. and Xue, QJ. Synthesis of ordered
mesoporous silicon oxycarbide monoliths via preceramic polymer nanocasting. Microporous
and Mesoporous Materials, 2012, 147 (1): 252-258.
51
Chapter 3
Preparation of Silicon Oxycarbide via Impregnation Using
Modified Layered Double Hydroxides as Templates
3.0 . Introduction
An efficient method to fabricate silicon oxycarbide ceramics is based on using a pre-
ceramic polymer. However, the interlayer distances of the LDHs - the template utilized in this
research - are normally small - less than 1.0 nm - which is too narrow of a space for most of
the polymer precursors that have been used to prepare SiOC ceramics to infiltrate. In the last
Chapter, we have utilized LDHs as sacrificial templates and a small molecular size pre-
ceramic precursor, namely tri-isopropylsilane (TPS), that is capable to infiltrate their structure
via chemical vapor deposition (CVD) to prepare, by controlled-temperature pyrolysis and
removal of the template, a SiOC ceramic with surface areas ranging from 330 m
2
/g to 540
m
2
/g and pore volumes from 0.50 cm
3
/g to 0.91 cm
3
/g (Yan at al., 2014). The use of CVD for
the preparation of microporous materials at large scale is challenging, however, requiring
expensive and complex reactor and control hardware, and controlling the final quality and
uniformity in properties of the produced materials is not a straightforward task.
In this Chapter, we propose instead a different fabrication method to prepare high surface
area and pore volume SiOC ceramics via the use of LDHs as templates. The technique
involves the infiltration of their structure by a blend of two pre-ceramic polymer precursors,
specifically hydridopolycarbosilane (HPCS) and allyl-subsituted HPCS (AHPCS). AHPCS
52
has been used previosuly (Deng at al., 2014; Elyassi at al., 2013; Xu et al., 2010) to prepare
SiC and SiOC microporous ceramics at relatively low temperatures of ~ 750
o
C. For example,
our group has prepared SiC microporous inorganic membranes via the dip-coating of thin
AHPCS films on macroporous SiC substrates (Deng et al., 2014; Elyassi et al., 2013) followed
by controlled-temperature pyrolysis. Xu et al. (2010) utilized AHPCS as the precursor
together with a micron size patterned silsesquioxane template powder in order to fabricate a
SiOC ceramic. We are not aware, however, of any other study prior to this one that reports the
preparation of high surface area microporous SiOC ceramics with a hierarchical pore structure
via the use of pre-ceramic polymers and LDHs as sacrificial templates.
Our method involves modifying the structure of the LDH template prior to its use via the
incorporation of a surfactant, namely sodium dodecylbenzenesulfonate (SDBS), by a re-
stacking method in order to enlarge the material’s interlayer distance (Costa at al., 2014). The
use of a blend of two pre-ceramic polymers HPCS and AHPCS with a very similar back-bone
but quite different molecular weights (the AHPCS has a large MW, ranging from 1000 to 5000
Dalton while the HPCS has a low MW that ranges from 132 to 352 Dalton) provides an added
degree of control in terms of being able to infiltrate the LDH structure with the desired
quantity/composition of the pre-ceramic polymer precursor. The overall preparation technique
is quite starightforward and well-suited for large-scale commercial application and materials
production and involves the direct impregnation of the SDBS-modified LDH (SDBS-LDH) by
the AHPCS/HPCS pre-ceramic precursor blend, followed by controlled-temperature pyrolysis
(see Experimental section below for further details). As we demonstrate in this paper, novel,
highly porous SiOC materials with high surface area, a multi-modal pore size distribution with
53
large micropore volume, and an interconnected pore space with slit-like pores are produced by
such a route.
The morphology, composition and structure of the resulting materials before and after
calcination were studied. The interlayer distance of the original LDH and the modified LDH
template were compared from XRD results. The final Si
x
C
y
O
z
materials both before
calcination and after calcination were characterized by XRD, XPS and SEM-EDX.
Characterization of the structure and the morphology of the porous materials both before
calcination and after calcination was carried out using scanning electron microscopy (SEM).
Nitrogen adsorption-desorption isotherms were also used to identify the specific surface area,
the pore size distribution, the pore volume and other characteristics of the resulting porous
materials under various preparation conditions.
3.1. Experimental
In this section we describe the procedure for the fabrication of the porous SiOC ceramic.
3.1.1. Materials
The AHPCS and HPCS (from Starfire Systems, Inc), SDBS (from Sigma-Aldrich Co.),
tetrahydrofuran (from VWR International), and hydrochloric acid (37 wt.%, from VWR
International) were used without further purification. The MgAl-LDH powder was provided
by Sasol Corporation (Sasol Mg 70 with a Mg/Al ratio of 3.0).
54
3.1.2. Preparation of SDBS-modified MgAl-LDH
As a first step, the fraction of the LDH powder that has a particle size less than 25 µm was
separated using a 500-mesh sieve. Next, the MgAl-LDH was calcined in a tubular furnace
under an argon atmosphere at 450
o
C for 3 h in order to convert it into metal oxide (Yang et al.,
2002). The calcined LDH was then dispersed in a 0.1M aqueous SDBS solution with a
solid/solution ratio of 1g/50 cm
3
(Costa et al., 2005). The mixture was then stirred by a
magnetic stirrer at ambient temperature for 24 h. It is well known that this procedure
reconstitutes the 2-D lamellar structure of the LDH while simultaneously allowing the
incorporation of the surfactant into its structure (Yang et al., 2002) – see further discussion
below. The resulting modified SDBS-LDH template was then filtered and dried at room
temperature.
3.1.3. Preparation of the silicon oxycarbide material
One gram of the SDBS-LDH was then impregnated with 0.35 ml of a AHPCS/HPCS/THF
solution (weight ratio of 1:2:2 for all materials presented here). The resulting composite
material was heated in a tubular furnace with a rate of 1
o
C /min according to the following
procedure (Yan at al., 2014; Elyassi at al., 2007) previously used by our group to prepare SiC
and SiOC ceramics: first, the temperature was raised to 200
o
C and held at this temperature for
1 h, then increased to 400
o
C and kept at this temperature for one more hour, and finally to 750
o
C and kept there for two additional hours while under an argon atmosphere; after that the
material was cooled down to room temperature with a rate of 5
o
C /min. The acid-leaching
process in order to remove the LDH sacrificial template from the resulting composite (Yan et
55
al., 2014) was carried out by immersing the material into a HCl solution (18.5 wt%) for two
days. The resulting black powder was washed several times with de-ionized water and then
dried in air for two days. The resulting SiOC ceramic was shown to contain free (elemental)
carbon – see discussion to follow. To remove the free carbon, the sample was calcined in a
muffle furnace under an air atmosphere at 450
o
C for 3 h, a procedure shown previously
effective to eliminate the extra carbon (Yan et al., 2014).
3.1.4. Characterization
Both the uncalcined and calcined samples were characterized by various surface techniques.
X-ray diffraction (XRD) analysis was carried out using a Rigaku X-ray diffractometer, with
the CuK
α
line used as the X-ray source with a monochromator, positioned in front of the
detector. Scanning was performed over a 2 θ angle range from 1° to 50° or from 10° to 90°
with a scanning rate of 4°/min and step rise of 0.05. X-ray photoelectron spectroscopy (XPS)
measurements were performed using a Thermo ESCALAB 250 spectrometer. A
monochromatic AlK
α
radiation source ( hν = 1486.6 eV) was used with a spot size of 400 μm.
Pass energies of 100 eV for the survey scan and 20 ev for the high-resolution scan were used
with the analyzer. Low-resolution survey scans were also acquired at binding energies
between 1 eV and 1400 eV with a resolution of 1 eV. The high-resolution scans of the C-1s
and Si-2p regions were acquired with a resolution of 0.05 eV. The data analysis was carried
out using the XPSPEAK Version 4.1 software. The samples, before and after sectioning, were
examined by a scanning electron microscopy (SEM) JEOL JSM 7001 instrument at room
temperature, and a 15 kV acceleration voltage. The samples were also analyzed using a JEOL
JSM 6360M SEM instrument equipped with an EDAX FALCON energy dispersive X-ray
56
spectrometry (EDX) unit. Before imaging, the SiOC powder was sprinkled onto a copper tape.
The sample was then sectioned using a JEOL cross-section polisher, utilizing an argon-ion
beam at 6 kV, at a milling rate of 100 micron/h for 25 min. Nitrogen adsorption-desorption
isotherms of the porous ceramic samples were measured with a Micromeritics ASAP 2010
adsorption analyzer at -196
o
C (77 K). Prior to the measurements, the porous SiOC material
was degassed overnight at 110
o
C. The Brunauer–Emmett–Teller (BET) method was utilized
to calculate the surface areas. The pore volume and pore-size distributions were derived from
the isotherms using the Barrett-Joyner-Halenda (BJH) as well as the Horvath-Kawazoe (HK)
methods.
3.2. Results and Discussion
The results are presented and discussed below:
The XRD patterns of the original MgAl-LDH and the modified SDBS-LDH are both
shown in Figure 3.1. Before its modification, the XRD pattern of the LDH exhibits the typical
peaks for MgAl-LDH with a lamellar distance of 0.76 nm, too small for the AHPCS/HPCS
polymer blend to penetrate into the LDH gallery (we have measured via Dynamic light
scattering (DLS) conducted with Dynapro Titan instrument the radius of gyration of a
AHPCS/THF=1:1 mixture to be 6.4 nm, that of a HPCS/THF=1:1 mixture to be 1.5 nm, and
that of the AHPCS: HPCS: THF=1:2:2 mixture to be 2.3 nm). After the LDH is modified by
SDBS, the (003) peak has shifted to lower angles, corresponding to an interlayer distance of
2.9 nm, indicating that the MgAl-LDH interlayer space has been successfully enlarged with
the SDBS intercalation and the LDH regeneration.
57
Figure 3.1: XRD spectra of the original LDH template and the LDH modified by SDBS,
SDBS-LDH.
The porous materials that were fabricated by using the AHPCS/HPCS polymer blend as
the precursor and the SDBS-LDH as the template were studied by XRD before and after the
calcination step to remove any potential free-standing carbon, as discussed in the
Experimental section. The resulting analysis data are shown in Figure 3.2. The XRD spectra
illustrate that both materials, before and after calcination, are fairly amorphous, indicating that
the final pyrolysis temperature of 750
o
C is probably not high enough to give rise to a
58
thoroughly crystallized Si
x
O
y
C
z
or SiC structure. The broad peaks between 21º - 24º are
typical for SiOC materials (Yuan et al., 2012), whereas those between 40º - 45º may be
interpreted as the representing both graphite carbon and β-SiC (Yuan et al., 2012; Elyassi et
al., 2008). The XRD spectra of both the uncalcined and calcined materials indicate that the
LDH template has been removed completely, since no typical peaks for MgO, Al
2
O
3
, or
MgAl
2
O
4
are detected in the spectrum.
Figure 3.2: XRD spectra of the samples prepared by impregnation before calcination (A)
and after calcination (B).
59
Table 3.1: XPS elemental composition of the Si
x
O
y
C
z
samples before cross-section
polishing. Also shown is the excess carbon.
Samples
Atomic ratio
Composition
Si O C
B 1 2.3 3.3 SiO
2.3
C
1.7
+C
1.6
A 1 1.7 0.2 SiO
1.7
C
0.2
XPS elemental compositions of the external surface of the SiO
y
C
z
material before and after
calcination are shown in Table 3.1. The typical formula for pure silicon oxycarbide materials
is reported to be SiO
4
-
x
C
x
(Tamayo et al., 2008), which means that the (O+C)/Si atomic ratio
is typically ~ 4. Before calcination, the atomic ratio of (O+C)/Si on the material’s external
surface was 5.6 indicating the potential presence of free carbon. After calcination of the
sample, the outer surface appears to be extensively oxidized, the material’s structure
corresponding SiO
1.7
C
0.2
. This is quite surprising, since silicon oxycarbide materials are
thought to possess substantial oxidation resistance. In fact, as noted in Chapter 2, the same
calcination technique when applied to the silicon oxycarbide materials prepared by us via
CVD of TPS using LDH as a sacrificial template was quite effective in removing the excess
carbon but left the silicon oxycarbide structure fairly unaffected: the structure shifted from
SiO
1.7
C
2.3
to a bit more oxidized state of SiO
1.9
C
2.1
.
60
Figure 3.3: SEM-EDX elemental composition of the sample after cross-section polishing,
but before calcination.
61
Figure 3.4: SEM-EDX elemental composition of the sample after cross-section polishing
and calcination.
To better understand the differences between the oxycarbide materials prepared with the
LDH as sacrificial templates using pre-ceramic polymer infiltration and those prepared via
CVD using TPS, sectioned oxycarbide particles prepared with and without calcination in air at
450
o
C were studied via SEM-EDX. The sensitivity of the SEM-EDX measurements is not as
good as that of XPS, so one cannot make direct comparisons between the chemical structures
generated by the two techniques. However, SED-EDX is more convenient to use for
visualizing and for providing an elemental map of the material’s internal structure. When
62
looking at Figure 3.3, for the as-prepared material without high-temperature air treatment, the
C:O ratio is quite spatially nonuniform ranging from 1.45 (area 1), to 2.1 (area 2), to 3.2 (area
3). The corresponding numbers for the calcined sample shown in Figure 3.4 are quite more
uniform spatially, specifically 0.9 (area 1), 1.1 (area 2), 0.75 (area 3), 1.1 (area 4) and 0.93
(area 5).
The picture that emerges when comparing the data in Figures 3.3 and 3.4 is consistent with
the XPS data in Table 3.1. Specifically, the as-prepared material contains free carbon which
appears to be non-uniformly distributed spatially. The air oxidation appears to be very
effective in removing the free-standing carbon. However, the interior of the particles has not
undergone the deep oxidation of the surface as indicated by the XPS data of Table 3.1.
Examining the EDX data in Figures 3.3 and 3.4, one may conclude that either a slight increase
of the O:Si ratio has taken place or no increase at all, if one ignores the result for area 3 in
Figure 3.3. This would tend to indicate then that the deeply-oxidized surface layer acts an
effective barrier for the further in-depth oxidation of the silicon oxycarbide material.
63
290 288 286 284 282 280
0
2000
4000
6000
8000
10000
12000
14000
C-O
C-C
C-Si
Intensity (a.u.)
Binding energy (ev)
Figure 3.5: High resolution XPS C-1S spectra of the sample prepared by impregnation before
calcination.
64
106 104 102 100 98
0
500
1000
1500
2000
2500
3000
SiO4
OxSiCy
Intensity (a.u.)
Binding energy (ev)
Figure 3.6: High resolution XPS Si-2P spectra of sample prepared by impregnation before
calcination.
In order to better understand the composition and bonding structure of the fabricated
material, high resolution XPS C-1s and Si-2p scans were obtained for the aforementioned
sample before calcination and are shown in Figures 3.5 and 3.6, correspondingly. These XPS
spectra are quite similar with those of the silicon oxycarbide materials prepared by us using
CVD of TPS and LDH sacrificial templates (Yan et al., 2014), for example, compare Figure
3.5 with Figure 3 of ref. (Yan et al., 2014), and Figure 3.6 with the corresponding Figure 4 in
the same paper. As previously discussed (Yan et al., 2014), in Figure 3.5 the very strong peak
at 283.4 ev corresponds to the Si-C bonds (Karakuscu et al., 2009), hence indicating that SiC
and SiOC are the dominant species in the material. The peak a 284.6 ev is due to the C-C
65
bonds, which is indicative of the presence of free carbon. A weak shoulder detected at 286.9
ev accounts for the C-O bonds, potentially from adsorbed CO
2
(Yuan et al., 2012), as also
noted previously (Yan et al., 2014).
Figure 3.6 that shows the high resolution Si-2p scans indicates the presence of the SiO
bond (the peak centered at 103.6 ev), whereas the three peaks at 102.9 ev, 102.3 ev and 101.8
ev are all typical for Si-C bonds that belong to [SiO
3
C
1
], [SiO
2
C
2
] and [SiOC
3
] units,
respectively (Karakuscu at al., 2009). The only difference between the scans in Figure 3.6 and
those in Figure 4 of ref. (Yan et al., 2014), is that the oxycarbide material prepared by CVD of
TPS contains an additional small peak indicative of the presence of [SiC
4
], which is absent for
the materials prepared by pre-ceramic polymer infiltration and pyrolysis. It is unlikely,
however, that the presence of that species explains the differences in the surface oxidation
behavior of these two materials, as the [SiC
4
] shoulder in the CVD material disappeared
following air calcination (e.g., compare Figures 4 and 6 in ref. [Yan et al., 2014]).
66
Figure 3.7: SEM images of the sample prepared by impregnation, before calcination. (a)
and (b) show the images before polishing, while the rest represent those after polishing.
67
Figure 3.8: Same as in Fig. 3.7, but after calcination.
68
The morphology of the material prepared by impregnation before calcination was studied
by SEM and is shown in Figure 3.7. The material is comprised of small particles with a
diameter mostly between 10 µm – 20 µm, which smaller that the particle size of the silicon
oxycarbide materials prepared by the CVD approach (Yan at al., 2014) Furthermore, the
particles are irregular in shape (while the particles for the CVD materials are quite spherical)
and appear to be interconnected with each other. This is probably due to preparation technique,
whereby the AHPCS/HPCS polymer blend inevitably adheres to the template particles’
external surface in addition to penetrating into the interlayer space, thus acting as the glue that
connects the particles together. After pyrolysis at 750
o
C and acid leaching, the ceramic that
forms bonds the porous particles together as shown in Figure 3.7. That the pre-ceramic
polymer indeed enters the interlayer space of the LDH template can be seen in Figure 3.7f.
The cross-sectional polishing of these particles reveals a well-aligned 2-D lamellar structure,
which is contributed by the space that the template leaves behind after it is removed. The
particles in Figure 3.7 reveal a hierarchical structure, consisting of micron size pores in
between the interconnected particles and in some of the particles themselves, and mesopores
in the particles themselves which are negative replicas of the template’s 2-D lamellar
framework. There is a third type of pores, however, which are too small to be visible in the
SEM images, but which are detected by the N
2
adsorption - desorption method. These
nanopores are formed during the pyrolysis of the pre-ceramic polymer to form the ceramic
from the gases that are emitted and from the “freezing” of the original polymer’s free-volume.
The morphology of the materials after air calcination is shown Figure 3.8. There are some
visible changes in the external shape of the particles, indicative of some shrinkage that may
happened during the calcination process, so that the structure is to some extent twisted.
69
However, the particle structure has not changed significantly, and its interior 2-D lamellar
structure is still quite intact; that the internal well-patterned structure is maintained is further
validated by the N
2
sorption measurements, discussed below.
Table 3.2: The surface area and pore volumes of the SiO
y
C
z
samples. A indicates the
sample before calcination, while B represents the sample after calcination.
Sample
S
BET
(m
2
/g)
V
total
(cm
3
/g)
V
meso
(cm
3
/g)
A 531 0.47 0.25
B 502 0.56 0.35
As noted above, nitrogen adsorption-desorption measurements were carried out in order to
study the surface area and pore size characteristics of the silicon oxycarbide materials
fabricated by pre-ceramic polymer pyrolysis with the aid of sacrificial LDH templates. Table
3.2 shows data of two silicon oxycarbide samples prepared under identical conditions. One of
these is the as-prepared sample, while the other is a sample that has undergone air calcination.
The as-prepared silicon oxycarbide material has a specific surface area of 531 m
2
/g, and a pore
volume equal to 0.47 cm
3
/g that is almost equally distributed among mesopores (0.25 cm
3
/g)
and micropores (0.22 cm
3
/g) – note that nitrogen adsorption is not capable to detect the
presence of macropores. Upon air calcination, the pore volume increases to 0.56 cm
3
/g as a
result of an increase of the mesopore volume from 0.25 cm
3
/g to 0.35 cm
3
/g, while at the same
time the micropore volume, remains relatively unchanged, a 0.01 cm
3
/g, or ~ 5% loss. This
indicates then that most of the free carbon occupies the 2-D lamellar negative replica of the
LDH template structure. The surface area decreases to 502 m
2
/g, probably as a result of the
slight reduction in the micropore volume.
70
Figure 3.9: Nitrogen adsorption-desorption isotherms of the samples fabricated by
impregnation, before (A) and after (B) calcination.
The sorption isotherms of the samples before and after calcination are reported in Figure
3.9. The steep increase at relatively low pressures corresponds to microporosity, whereas the
hysteresis loop between relative pressures of 0.4 – 0.8 is associated with the mesopores. The
hysteresis loop is characteristic of type IV porous materials with a subtype H4 isotherm, which
is indicative of the presence of narrow slit-like pores (Sing et al., 1985). This observation
provides additional evidence that the material, templated using the SDBS-LDH, has at least
partially replicated the template’s layered structure.
71
Figure 3.10: The HK pore size distribution of the samples fabricated by impregnation,
before (A) and after (B) calcination.
72
Figure 3.11: The BJH pore size distribution of the samples fabricated by impregnation,
before (A) and after (B) calcination.
Figure 3.10 presents the micropore size distributions, obtained by the HK method, for the
as-prepared and the calcined samples, while Figure 3.11 presents the mesopore size
distributions for the same samples, obtained by the BJH method. Both samples, before and
after calcination, demonstrate similar narrow pore size distributions, and the gain in mesopore
volume due to the removal of free carbon is clear in Figure 3.11. Note, furthermore, that the
average pore diameter in the mesopore region is 4.1 nm, not far removed for the interlayer
distance of the SDBS-LDH as estimated by XRD (Figure 3.1), particularly when one takes
73
into account the uncertainties involved in the estimation of pore diameters via the N
2
adsorption technique.
3.3. Conclusions
A simple process was proposed for fabricating highly porous silicon oxycarbide materials
with a hierarchical pore structure that utilizes an AHPCS/HPCS polymer blend as the
precursor and layer double hydroxides as the sacrificial templates. The method involves first
expanding the interlayer LDH region via the use of a surfactant followed by impregnation of
the template’s structure by the pre-ceramic polymer. The resulting composite material is then
converted into the ceramic via controlled-temperature pyrolysis followed by removal of the
sacrificial template as the final step that generates the material’s hierarchical pore structure.
An important aspect of the preparation method is the use of the HPCS/AHPCS blend that
allows for the complete infiltration of the template’s pore structure. The as-prepared material
contains some free carbon which can be conveniently removed via an air oxidation step which
increases the mesopore volume quite substantially while causing relatively small, ~ 5%,
decreases in the surface area and micropore volume, while maintaining the material’s
hierarchical pore structure and the 2-D lamellar mesopore environment.
The silicon oxycarbide porous material produced by the proposed technique has a high
surface area, an interconnected pore space with a multi-modal pore size distribution and high-
temperature stability. It is produced using a relatively low-cost commercial pre-ceramic
polymer precursor and a commercial low-cost sacrificial hard template via a convenient
synthesis procedure that can be readily adapted for large-scale production. The resulting
materials show good promise for use in the field of catalysis, for gas adsorption/separation
74
applications under harsh conditions, and as integral components in biomedical devices, etc.
We hope to report on some of these applications in the near future.
75
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77
Chapter 4
Future Work
In previous Chapters, silicon oxycarbide materials have been fabricated with layered
double hydroxide as a sacrificial template. There are several interesting aspects of the work
that remain incomplete that could be studied in future research projects, as discussed briefly
below.
Nanoporous silicon oxycarbide materials can be templated with layer double hydroxides
with different structures. The morphology of LDHs can be controlled, for example, into cone-
shaped (Dutta et al., 2013), stacked-disk-shaped (Tashi Yokoi et al., 2014), or can be prepared
into a thin film (Lei et al., 2012), etc. If a polymer precursor is mixed with LDH templates
with different morphology, the final silicon oxycarbide materials after pyrolysis and acid
leaching can be shape-oriented, which, will be useful in select application fields. Silicon
oxycarbide films can be prepared, for example, from silicon carbide substrate supported LDH
films by the templating method.
Nanoporous silicon oxycarbide materials have potential for application as catalyst supports,
catalyst, lithium battery anodes, gas absorbents, for gas separations, as bio-functioning
materials, etc. The materials which have been prepared previously in this research may be
useful in such applications and should be tested, as detailed further below.
The obtained silicon oxycarbide materials can be tested for gas adsorption, such as H
2
adsorption. Since the materials fabricated in this research show very high surface area of over
78
500 m
2
/g, the adsorbed gas amounts could be promising. One factor need to be considered is
that the porous materials surface will inevitably adsorb O
2
. The adsorbed gas will decrease the
sorption abilities for other gases. Thus, it will be critical to keep the fresh silicon oxycarbide
materials under relatively inert atmosphere prior to their use.
Catalysts with both mesopores and micropores have important practical value in the
modern chemical industry. The materials which have been designed in this research have high
surface area which is enough for gas or liquid transportation and active site attachment,
hierarchical pore size distributions in both the meso- and micropore range which provide
channels for the reactants to pass through; in addition, they have high temperature stability,
which is important for all high temperature reactions, chemical stability that makes their use as
catalysts ideal for reactions in harsh environments, relatively low cost since they are prepared
via convenient CVD or impregnation procedures and commercial precursors. Thus, the
materials meet all the requirements for a good catalyst or catalyst support. Active metals like
Au, Pt, Cu can be deposited on the silicon oxycarbide particles’ pore surface by impregnation
or colloidal deposition to form catalysts for reactions such as the water gas shift (WGS).
Specific surface area, the pore structure and mechanical strength are tunable according to the
reaction requirements.
Silicon oxycarbide is also attractive for application as anodes for lithium batteries. The
materials fabricated in this research, which have ordered layered structure, can be considered
as a possible substitute for the current lithium battery anode materials. Therefore, these
materials, with various surface area and pore structures properties, can be pressed into
electrode for lithiation capacity testing. Finally the simulation of synergistic effects of the
silicon oxycarbide precursors and the LDH template on the porous materials’ properties will
79
be an attractive and meaningful study. Since LDH templates are not stable during the pyrolysis
procedure but meanwhile the ordered slit-like structure has been maintained during pyrolysis
before the LDHs have been eliminated, it will be necessary to study the pore structure
evolution of the precursor and template composites during pyrolysis. This study will be
important to guide future work to prepare materials with the desired pore structure
characteristics.
80
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Abstract (if available)
Abstract
Due to their promising mechanical properties, chemical durability, excellent oxidation resistance, high‐temperature stability (up to 1300 ℃) and other excellent properties, silicon oxycarbide (SixOyCz) materials are being used today as catalysts, catalytic supports, gas or liquid adsorbents, lithium battery anodes, light emitters, and blood contact agents. ❧ In this research,layered double hydroxides (MgAl‐LDHs), or modified LDHs (SDBS‐LDHs) are used as templates to fabricate nanoporous SixOyCz materials using precursors, such as tri‐isopropylsilane (TPS) and a allyl hydridopolycarbosilane (AHPCS)/hydridopolycarbosilane (HPCS) mixture, with synthesis methods that include chemical vapor deposition (CVD) and direct impregnation. The preparation conditions such as the pyrolysis temperature and the CVD precursor injection flow rates are investigated. We study the characteristics of the templated porous materials fabricated by different preparation routes using techniques such as small‐angle X‐ray diffraction (XRD), wide‐angle XRD, X‐ray photoelectron spectroscopy (XPS), XPS elemental analysis, SEM‐Energy dispersive X‐ray spectroscopy (SEM‐EDX), scanning and transmission electron microscopies (SEM and TEM), as well as nitrogen adsorption‐desorption (BET) measurements. ❧ The porous SixOyCz materials fabricated from the TPS precursor and the unmodified LDH template by CVD have a specific surface area as high as 541 m²/g and a total pore volume as large as 0.91 cm³/g. The materials consist of fairly uniform size hollow spheres with sizes between 10 and 30 µm and one opening, with an interior ordered layered structure and a bimodal pore size distribution, both in the mesopore and micropore regions. The template’s ordered structure has been successfully reproduced by the CVD procedure. After calcination of the materials in air at 450 ℃ for 3 h, most of the free carbon present is eliminated and any silicon carbide present initially has been oxidized into silicon oxycarbide. ❧ A convenient impregnation technique has also been selected to synthesize SixOyCz materials using AHPCS/HPCS polymer blends as precursors and SDBS‐LDH as a sacrificial template. The produced materials demonstrate a well‐aligned, slit‐like pore structure, a surface area of 531 m²/g with a considerable fraction of micropores and mesopores. The key aspect of the preparation process is the stabilization of the HPCS by the AHPCS during the thermal treatment, which guarantees an interconnected porous space in the final material. Calcination in air at 450 ℃ for 3 h removes any free carbon from the material, while the pore structure is preserved. ❧ The SixOyCz materials fabricated in this research, using economically-feasible templates and precursors and convenient synthesis procedures, have desirable structures, and show promise for applications such as catalyst supports, sorbents, membrane films for water filtration and gas separations, and as battery anodes, etc.
Linked assets
University of Southern California Dissertations and Theses
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Asset Metadata
Creator
Yan, Xiaojie
(author)
Core Title
Fabrication of nanoporous silicon oxycarbide materials via a sacrificial template technique
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Chemical Engineering
Publication Date
07/10/2014
Defense Date
06/18/2014
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
hierarchical,high surface area,layered structure,LDH template,OAI-PMH Harvest,silicon oxycarbide
Format
application/pdf
(imt)
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Sahimi, Muhammad (
committee chair
), Tsotsis, Theodore T. (
committee member
)
Creator Email
xiaojiey@usc.edu,yanxiaojie19@hotmail.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-c3-435754
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UC11287706
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etd-YanXiaojie-2649.pdf (filename),usctheses-c3-435754 (legacy record id)
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etd-YanXiaojie-2649.pdf
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435754
Document Type
Dissertation
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Yan, Xiaojie
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
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The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...
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Repository Location
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Tags
hierarchical
high surface area
layered structure
LDH template
silicon oxycarbide